HNBs (also referred to as femto-cells) are small transceivers which will be placed in customer's homes, workplaces or other locations in order to increase network coverage and to offload signalling and user plane traffic from network macro-cells. In order to make the installation of a HNB attractive to a customer, HNBs are expected to be offered with preferential calling rates and are also expected to provide additional services not provided by a legacy CS macrocell. It is anticipated that more and more customers will switch from fixed landlines to mobile phone usage at home or in the workplace, resulting in a significant increase in circuit-switched (CS) voice traffic over the mobile communication networks. In order to avoid needing to upgrade legacy mobile switching centres (MSCs) and CS core networks in view of this increase in demand, the possibility of offloading the additional CS voice traffic via the HNB to the IP Multimedia Subsystem (IMS) is being discussed by the 3GPP, with various architectural alternatives being elaborated on in TR 23.832 “IMS Aspects of Architecture for Home NodeB”.
By the time Home NodeBs are deployed, it is expected that User Equipments (UEs) having IMS clients and supporting handover between the IMS and the CS domain using Single Radio Voice Call Continuity (commonly known as SRVCC) will also be available. UEs having IMS clients are expected to be configured to prefer IMS VoIP when within coverage of an HNB, as HNBs are expected to provide one or a few UEs with good enough QoS to provide Voice services via the IMS.
At the same time, HNBs are also expected to serve UEs which do not have an IMS client (e.g. pre-release 9 legacy UEs), which use normal CS procedures to establish voice calls. The Inter-Working Function (IWF) in the Home NodeB is expected to “translate” the CS signalling (CS 24.008 Call Control and the related user plane media) from such UEs to IMS signalling and VoIP traffic. The result of this approach is that from the core network's point of view the call is handled as an IMS PS call regardless of whether the UE supports PS voice call functionality (i.e. whether it is IMS capable or not).
As discussed above, the HNB IMS subsystem is to act like an adaptor between the CS and PS user planes and call control functions. The whole IMS HNB subsystem is expected to be transparent to the core network. For a practical deployment, it is likely that the Macrocell(s) surrounding the HNB IMS subsystem will be incapable of supporting PS voice traffic with the same QoS provided by the HNBs. It is therefore preferred that if the UE has Single Radio Voice Call Continuity (SRVCC) capability then the handout from HNB coverage to outside macrocell is based on the SRVCC procedures defined in 3GPP TS 23.216 “Single Radio VCC; Stage 2”. At the moment there is no mechanism which allows the handover of a call leg with a legacy UE not having SRVCC capability while in the same network also allowing an SRVCC handover to occur for UEs with SRVCC capability. Without such a mechanism, either:                the UEs which are IMS capable will have to use CS when they are in IMS HNB coverage (i.e. UEs cannot use VoIP/SRVCC even if they are capable of doing so) unless the adjacent macrocells are upgraded to support VoIP; or        the CN has to implement different handout procedures based on the capabilities of the UEs.        
The present invention addresses the above problem. Aspects of the present invention are set out in the accompanying claims.